Added ability to customize thumbnailing engine.

Easy_thumbnails and sorl-thumbnail are supported out of the box but one could also easily provide their own by inheriting from ThumbnailEngine.

Previously it was assumed that it'd only work with easy_thumbnails. Now this can be customized, although easy_thumbnails is still used by default for backwards compatibility reasons.

What do you think?
Is it tested and confirmed that people can easily use their own custom `ThumbnailEngine` subclass? If this setting points to `foo.Engine`, and `foo` imports `adminboost.preview.ThumbnailEngine` (to subclass), will that trigger a circular import?